At one time, I had 20" summers and 18" winters. Both with RFT. Yes, the ride was softer on the 18" and my wife preferred them. I definitely did not as felt like I'd lost some of the handling and the X5 felt more like just another SUV. The 19" wheels may be a little better than the 18".

I got tired of the way the 18" wheels didn't full the wheel arches (as covered by Hammer above). I felt I had two choices:
1. Use spacers on rears. Too many people say negative things about doing that though.
2. Buy another set of 20" wheels and get rid of the 18" winters. This is what I did.

Yes, you could move to non-RFT and the ride will supposedly be better. Definitely quieter. I'll let others, who've gone this route, comment on this option.

You should be able to sell your 20" RFT tires pretty easily; especially if you sell them while still relatively new. Most people, like me, are going to stick with RFT...